1. Field of the Invention
The present invention relates to a diaphragm employed in a capacitance type loudspeaker. More particularly, the present invention relates to a diaphragm for a capacitance type loudspeaker using a high molecular film.
2. Background of the Invention
A capacitance type loudspeaker has a conductive film-like diaphragm provided between two fixed electrodes. The capacitance type loudspeaker utilizes the following sounding mechanism. A DC high voltage is applied as a bias between the film-like diaphragm and the fixed electrodes to form a strong electric field, and an AC electric field by an audio signal voltage is added to be superposed thereon, thus generating an electrostatic vibratory force to the diaphragm. This vibratory force causes the diaphragm to vibrate and hence generate sounds.
Consequently, as is apparent from the above-described structure and sounding mechanism, the property and conductivity of the diaphragm are critical for satisfactory performance. Therefore, it has been a conventional practice to vapor-deposit a metal, such as aluminum, or apply a surface active agent used for electrostatic prevention, onto the high molecular film for providing optimum conductivity to the diaphragm.
However, the conventional practice accompanies the following inconvenience.
First, in the capacitance type loudspeaker, the vibratory force F generated to the diaphragm is expressed by F .alpha.eQ/d.sub.1 +d.sub.2, with e denoting the audio signal voltage, Q denoting the charge of the diaphragm, and d.sub.1, d.sub.2 denoting the cavities between the diaphragm and the fixed electrodes. Practically, the required charge Q of the diaphragm is provided by applying a bias voltage E between the diaphragm and the fixed electrodes through a high resistance R.sub.0.
It is necessary that the minimum value of resistance R.sub.0 at this point is a constant when it satisfies R.sub.0 C.sub.0 &gt;1/2f.sub.0 with f.sub.0 denoting the lowest frequency of the loudspeaker and C.sub.0 denoting the capacitance between the fixed electrodes and the diaphragm. For instance, R.sub.0 &gt;40 M.OMEGA. is preferred under the conditions of f.sub.0 =25 Hz, C.sub.0 =500 pF.
Accordingly, if a conductor of the diaphragm is a good conductive material having a metal, such as aluminum, vapor-deposited thereon, a high resistor of R.sub.0 &gt;40 M.OMEGA. is required. Since a practically used capacitance type loudspeaker requires a bias voltage E of approximately 5000 V, a high resistor which can be safe at such a high voltage needs to be large. In addition, the resistor charged with a high voltage is extremely hazardous.
Meanwhile, the Japanese Patent Examined (KOKOKU) Publication No. 41-18646 (JP-B-41 18646) discloses a technique of achieving high resistance of the conductive layer of the diaphragm by applying a high resistance conductive material, such as a surface active agent for electrostatic prevention, onto the high molecular film, without requiring the high resistor in the good conductor.
However, the surface active agent has conductivity which is likely to be affected by ambient humidity, and has surface resistivity greatly changed by approximately 102 to 104 times.
In view of the above status of the art, forming a conductive high molecular membrane on the high molecular film surface is considered.
As a method of forming the conductive high polymer on the high molecular film surface, the Japanese Laid-Open (KOKAI) Patent Publication No. 63-20361 (JP-A-63 20361) proposes a method of forming a polymer layer by applying a solution of high molecular resin as a binder material containing a chemical oxidizer or a conductive high molecular monomer, then soaking the former in a solution containing the conductive high molecular monomer or exposing the former into a vapor of the conductive high molecular monomer, and soaking the latter in a solution containing the chemical oxidizer.
However, the above method requires an advanced technique of applying the thin membrane on the high molecular film. It also has a problem such that the high molecular resin used as the binder greatly changes the property of the high molecular film.
The above-described techniques leave the problems unsolved, such as the need of a high resistor, the effect of the ambient humidity and the effect on the property of the high molecular film forming the diaphragm. Solutions to these problems are now required.